In a notebook computer (hereinafter, referred to as a laptop PC), a DC voltage output from an AC/DC adapter is converted into a predetermined DC voltage by a DC/DC converter. The laptop PC is equipped with a battery charger for charging a rechargeable battery. The AC/DC adapter, the DC/DC converter, and the battery charger are also referred to as a switching regulator because they are equipped with a switching element capable of regulating an input DC voltage to output a predetermined DC voltage.
The switching regulator is configured to control either one of ON period and OFF period of one cycle period of the switching element in an operation mode such as, for example, a PWM (Pulse Width Modulation) mode and a PFM (Pulse Frequency Modulation) mode to output a constant DC voltage. Specifically, in the PWM mode, the ON periods are controlled while maintaining the switching frequency (hereinafter, referred to as an operating frequency) which corresponds to the frequency of turning ON/OFF the switching element constant, whereas in the PFM mode, the OFF periods are controlled by maintaining the ON periods constant, thereby controlling the average secondary-side voltage at a predetermined range. The PWM mode is used in many switching regulators because its constant operating frequency allows an easy noise-reduction filter design; it however has drawbacks in that its switching loss under light load state is large.
An operation mode called an intermittent mode is employed in order to compensate for the drawbacks of the PWM mode under a light load state. The intermittent mode is also referred to as a skip mode, a burst mode, or a sleep mode, for example. A switching regulator employing the intermittent mode monitors a load current or a load voltage and based on the monitored load state, operates in a PWM mode under a heavy load state or in an intermittent mode under a light load state. In the intermittent mode, the switching loss can be reduced by lowering the operating frequency from that in the PWM mode.
In the intermittent mode, the operating frequency is lowered as the load decreases, and a constant output voltage can be generated even until a no-load state. In this case, when the load decreases, the operating frequency of the switching element becomes lower than 20 kHz, which is the upper limit of an audible frequency. Under the no-load state, a very small current, which is determined by the circuit constants of capacitors or resistors contained in the power supply unit, will flow through the switching element, and the operating frequency will decrease to about 10 Hz. Therefore, under the light load state, inductors and capacitors constituting the switching regulator may vibrate at the audible frequency and thus generate noise. The causes of the noise generation are related to the physical arrangement, mounting method, and securing force of constituent elements of the switching regulator with respect to a board. Moreover, the causes are also related to the individual difference such as materials or capacity of the elements and the ambient temperature. Furthermore, since the noise generates due to a complexity of the causes and moreover the elements deteriorate over time, it is practically difficult to perfectly control the noise and even guarantee such a noise-controlled state for a long period of time.
Japanese Patent Application Laid-Open No. 2006-333572 describes a technique that detects noise in a vehicle to control a carrier frequency of a PWM-controlled converter. In the invention described in the above document, the noise within the vehicle is detected based on the wind sound or road noise generating accompanied with the driving of the car. Based on the detection result, the carrier frequency is lowered so as to improve the power conversion efficiency, for example, during high-speed driving where the detected noise level increases enough to kill the noisy sound resulting from the carrier frequency of the PWM-controlled converter. On the other hand, for example, during low-speed driving where the detected noise level decreases and hence the noisy sound resulting from the carrier frequency becomes relatively high, the carriage frequency of a switching portion is increased so as to suppress the noisy sound.
Japanese Patent Application Laid-Open No. 2002-095251 describes a switching regulator employing such an ON/OFF control that regulates the current limit of the switching regulator so as to reduce audible noise under the light load state. The switching regulator according to the invention described in the above document includes a state machine that regulates the current limit of the switching regulator based on the patterns of the feedback signal values supplied from the output of a power supply with respect to N cycles of preceding driving signals. The state machine regulates to further lower the current limit under the light load state so that the cycles are not skipped in order to reduce the operating frequency of the switching regulator to be within an audible frequency range until the density of the magnetic flux passing through a transformer becomes sufficiently small and generation of the audible noise is suppressed.
Japanese Patent Application Laid-Open No. 2002-058239 describes a switching regulator that is equipped with an auto-skip circuit capable of detecting whether the present load state corresponds to a light load state or a heavy load state. The switching regulator is configured to operate in a PWM mode under the heavy load state and, under the light load state, operate in a skip mode where the switching frequency of a main output transistor becomes lower; however, the switching regulator does not transition to the skip mode until the light load state continues for a predetermined period of time even when the load state has changed from the heavy load state to the light load state.